The present invention relates to mixers for superheterodyne communication receivers such as television receivers. More particularly, the invention relates to the use of a field effect transistor as the mixing element in a varactor tuner for a television receiver.
Field effect transistors have desireable characteristics for use as the mixing element in a mixer. The high conversion gain of FETs, their near square law transfer characteristic and extended range of operation as compared with a bipolar transistor has won widespread use of FETs in mixers of mechanical tuners. One drawback of the FET as a mixer, however, is that when it is biased to achieve more linear operation to minimize cross modulation and intermodulation products, it requires an oscillator injection voltage in the range of 0.5 volt to 1.0 volt in order to achieve good conversion gain. In a varactor tuner, however, due to the large variation in Q between the low frequency channels and the high frequency channels, the oscillator injection voltage ranges from 0.1 volt to 0.6 volt. This large swing and the low oscillator injection voltage renders satisfactory operation with any single operating point impossible.
It is accordingly an object of the present invention to utilize a FET as the mixing element in a varactor tuner.
Another object of the present invention is to provide adjustable bias control for a FET mixer to establish optimum operating points for various ones of the channels in the band being tuned.
Bias adjustment can be achieved in many ways, but in a varactor tuner it was discovered that changing the bias voltage to the FET also changed the input and output capacitance of the FET, causing detuning and/or variable loading of the various tuned circuits in the RF amplifier, the local oscillator, and in the mixer circuit itself.
It is accordingly another object of the present invention to provide a bias controlled FET mixer in which the bias voltage is held substantially constant while drain current is varied.
These objects are generally achieved by the selection of a high conversion gain field effect transistor such as a dual gate MOSFET. A resistor network connected to the gate and source electrodes thereof establishes the DC voltage bias and the drain current for the FET for one channel, such as channel 2, at which channel the varactor tuned oscillator provides low oscillator injection voltage. The resistor network is proportioned to provide optimum conversion gain by the FET mixer for this channel. A transistor is connected to the source electrode of the FET and is responsive to the tuning voltage that is applied to the varactor diode tuned circuits to change the drain current of the FET as the tuning voltage changes. The tuning voltage is proportional to the oscillator injection voltage. The resistor network holds the bias voltage substantially constant while the drain current is varied to achieve the desired operating point for the FET for each of the channels in the band.